This invention relates to removal of suspended substances, surface active substances, heavy metals, etc. from waste water containing said extraneous substances, said removal being effected by forming bubbles in said waste water and thereby causing said extraneous substances to be deposited on said bubbles by virtue of adsorption or adhesion. More particularly, this invention relates to an apparatus and method for continuous froth flotation, whereby said removal of extraneous substances is effected with enhanced efficiency.
In recent years, pollution due to the industrial waste has come to pose a serious social problem. In particular, suspended substances, surface active substances, heavy metals and the like contained in waste water present serious problems. Thus, need is felt for research directed to the development of techniques for the separation and concentration of suspended substances and surface active substances contained in waste water and techniques for the removal of heavy metals also contained in the waste water. In addition, further improvement are required in the existing techniques employed for the separation and concentration of metals contained in sea water as one aspect of ocean development and techniques employed for the separation and concentration of substances mixed or dissolved in minute amounts in solutions. As means capable of fulfilling these requirements, the flotation method and the foam separation method which accomplish desired concentration and separation of suspended substances and surface active substances by utilizing said substances' ability to form stable froth and the ion flotation method which accomplishes desired concentration of substances devoid of surface activity by imparting surface activity thereto and thereby forming froth therein are drawing much interest because of the simplicity of the construction of the apparatus adapted to effect these methods and the conspicuousness of the effect of treatment attained.
The apparatus of this sort which have to date been put to actual use for the flotation are invariably of a simple construction composed of a tank which is provided at the lower portion thereof with a gas dispersing unit designed to form bubbles. Improvements have merely been given to the shape of said tank, to the positions at which the froth outlet, the raw water feed inlet, the treated water outlet, etc. are attached, to the type and shape of said gas dispersing unit, to the number of gas dispersing units to be installed, and so on. In these apparatus, the entire volume of the liquid contained in the tank is evenly agitated by the upward current of bubbles and the consequent phenomenon of air lift and the liquid under treatment is caused to flow in a downward current via the shortest path. Thus, almost no concentration gradient is recognized to exist from the upper through lower portions of the tank interior. This is equivalent to saying that the entire volume of the liquid under treatment is not brought into sufficiently uniform contact with the bubbles and the effect of separation, therefore, is deficient. When the effect of separation reaches a certain degree, it always levels off and cannot be brought to a satisfactory level. Where better effect of separation is desired, therefore, the same treatment must be repeated a plurality of times by using a plurality of similar apparatus.
These known techniques have been disclosed by in British Pat. Nos. 955,321, 1,062,346 and 1,074,945 and U.S. Pat. Nos. 3,032,199 and 3,203,893, for example.
An object of the present invention is to provide an apparatus and method for continuous froth flotation which overcomes the aforementioned disadvantages suffered by the conventional apparatus and effects the flotation and separation of froth with extremely high efficiency.